1. Field of the Invention
The present invention relates to an engine preheating system for controlling a heating member to quickly preheat an engine such as a diesel engine.
2. Description of the Prior Art
To start a diesel engine quickly in cold climate, an electric current is supplied from a battery to a heating member such as a glow plug to preheat the same, for assisting in starting the engine. Recent years have seen the development of a self-temperature-controlling glow plug. Such a self-temperature-controlling glow plug is used to start a diesel engine quickly. More specifically, when a keyswitch associated with the engine is turned on, a large current is supplied to the glow plug to quickly preheat the same within a short period of time. Alternatively, when a quick-acting afterglow function is to be performed after the keyswitch is turned off, a large current is supplied to the glow plug to quickly preheat the same for stabilizing engine idling immediately after the engine is started.
One self-temperature-controlling glow plug is disclosed in Japanese Patent Application No. 58(1983)-182459. The disclosed glow plug comprises a heating coil and a piece of magnesium oxide which are inserted in a sheath which is resistant to heat and corrosion. The heating coil comprises two coil elements having different resistance temperature coefficients, the coil elements being connected in series with each other. These heating coil elements are referred to a rush coil and a brake coil, respectively, which are named after the functions to be performed thereby. The rush coil is disposed in a front end portion of the sheath, and the brake coil is disposed in a rear end portion of the sheath. The resistance temperature coefficient of the rush coil is constant irrespective of the temperature, but the resistance temperature coefficient of the brake coil is higher as the temperature becomes higher.
When the self-temperature-controlling glow plug starts to be preheated, the rush coil element is first heated red quickly. If the quickly heated condition continued for a long time, the temperature of the glow plug would become higher than necessary, resulting in a coil breakage. To avoid this, the resistance of the brake coil element is increased with the temperature rise, thereby reducing the supplied current. As a result, the temperature of the glow plug is prevented from rising excessively, but is kept at a preset level.
When the self-temperature-controlling glow plug is employed in an engine preheating system, it is preferable to supply a large current to the rush coil within a short period of time when the glow plug begins to be preheated, so that the glow plug will quickly be heated to a preset temperature of such as 800.degree. C., for example. However, the batteries on general motor vehicles cannot supply such a large current when starting to preheat the glow plug.